1. Field of the Invention
This invention relates to deasphalting and catalytic cracking. More particularly, it relates to a process for obtaining a hydrocarbon oil with a low asphalt content by solvent deasphalting an asphalt-containing hydrocarbon feedstock with a liquid solvent, and catalytic cracking of the resulting deasphalted oil.
2. Description of the Prior Art
Many petroleum crude oils contain significant quantities of asphalt. Asphalts have a boiling range which coincides with that of many of the higher boiling constituents of petroleum. Since asphalts readily oxidize to form carbon and sludge, their presence is undesirable in lubricating oils. Further, due to their high coking propensity, asphalts must be excluded from catalytic cracking units where high coke levels are detrimental to catalyst performance.
Solvent deasphalting has proven effective in providing low asphalt-content petroleum fractions and has been practiced commercially for many years. In these deasphalting processes, the oil dissolves in the selected solvent while the asphalt, which is present in a dispersed state in the mineral oil, precipitates during the solvent treatment. Propane deasphalting has proven to be one of the most commercially successful of these processes, especially in the preparation of high quality lubricating oils.
The prior art is replete with solvent deasphalting processes employing a variety of solvents and solvent mixtures. Typical of this prior art is U.S. Pat. No. 2,337,448 of Carr which discloses a process for deasphalting a heavy residuum by contacting it at elevated temperatures with a deasphalting solvent such as ethane, ethylene, propane, propylene, butane, butylene, isobutane or mixtures thereof. A number of other solvents and solvent combinations are disclosed in the patent art as being useful in solvent deasphalting, including a two or three component solvent selected from hydrogen sulfide, carbon dioxide and C.sub.3 -C.sub.5 hydrocarbons (U.S. Pat. No. 4,191,639 of Audeh et al), propylene-acetone (U.S. Pat. No. 3,975,396 of Bushnell et al) and naphtha or C.sub.3 -C.sub.5 hydrocarbons together with small amounts of ethane, ethylene, alcohols, esters or ketones (U.S. Pat. No. 2,045,742 of Winning et al). U.S. Pat. Nos. 3,206,388 and 3,228,870 of Pitchford disclose the effectiveness of n-propyl alcohol or isopropyl alcohol containing a small quantity of water or a larger quantity of C.sub.5 -C.sub.15 n-paraffin as a deasphalting solvent for either a crude oil or a fraction thereof.
Bray et al (U.S. Pat. Nos. 2,081,473 and 2,101,308) and Bray (U.S. Pat. No. 1,949,989) teach a wide range of solvents that will dissolve the oil and any wax in the oil but will not dissolve the asphalt. This extensive list includes liquified normally gaseous C.sub.2 -C.sub.4 hydrocarbons, naphtha, and casinghead gasoline, as well as alcohol, ether, mixtures of alcohol and ether, acetone and the like. Only the preferred liquified C.sub.2 -C.sub.4 hydrocarbons are exemplified, however.
C.sub.1 -C.sub.4 alcohols were employed in U.S. Pat. No. 3,364,138 of Van Lookeren Campagne to remove the resins from an oil-solvent solution after the asphalt had been precipitated from a residual petroleum stock by propane. Solvent extraction of the resins from asphalt by the use of alcohols was also the subject of U.S. Pat. No. 3,003,946 of Garwin (C.sub.3 -C.sub.4 aliphatic alcohols) and U.S. Pat. No. 2,725,192 of Kieras (n-butanol).
U.S. Pat. No. 4,548,711 Coombs et al, teaches the benefit of supercritical extraction of heavy crudes and resids in a segmented baffle tray extraction column. Solvent/feed ratios of 2.5/1 to 4.5/1 by weight offered significant advantages over the prior art solvent/feed ratios of between 7.5/1 to 10/1 by weight.
U.S. Pat. No. 4,565,623 Davis, taught deasphalting oils using miscible solvent (at a low ratio) and a carbon dioxide anti solvent. The solvents used were C.sub.4 -C.sub.12 aliphatic hydrocarbons or toluene.
In U.S. Pat. No. 4,592,831, Rhoe et al used ratios of solvent to residue of at least 2:1 no greater than 4:1.
All of these processes have one thing in common, they all try to recover most of the solvent for reuse in the process. This is because the solvents used are moderately expensive and must be recovered to permit economical operation of the deasphalting process.
Some attempts were made to simply add asphaltene containing heavy feeds to catalytic cracking units. The primary difficulty with processing these heavy feeds such as resids in catalytic cracking units was the large problem created by the relatively small amount of asphaltenes, and metals which were concentrated in the asphaltene fraction. If we could "squeeze" out only these most refractory and difficult components, we could efficiently upgrade the other heavy materials in the resid fractions in the catalytic cracker. We knew the presence of minor amounts of hydrocarbon solvent would not degrade the cat cracker operation, and may even be beneficial. We knew the catalytic cracking process generates a spectrum of lighter hydrocarbon products, including naptha boiling range materials and olefinic hydrocarbons, in the C.sub.5 -C.sub.10 range. These materials are ideal solvents for selective rejection of asphaltenes.
We realized that for efficient upgrading of heavy, metals laden crude, it was necessary to take a different approach. The prior art deasphalting process did a good job, but cost too much, in terms of crude loss and capital. Rather than do everything in the solvent deasphalting unit, or add resid to the catalytic cracking unit, we discovered that by intentionally doing a poor deasphalting job, using a process derived solvent, we could drastically reduce the capital and operating cost of deasphalting. By close coupling a poor deasphalting process with a conventional catalytic cracking process, we could achieve demetalation without excessive yield loss in the deasphalter, and efficiently crack the resid without destroying the cracking catalyst.